A cement job is performed when the drilling has lowered the casing in the well-bore. A cement job is the process of mixing a slurry of cement, cement additives and water and pumping it down through casing to critical points in the annulus around the casing or in the open hole below the casing string. Cement supports and protects well casings and helps achieve zonal isolation. Critical to safer, environmentally sound, and profitable wells, zonal isolation is created and maintained in the wellbore by the cementing process. The cement is deigned based on various well parameters, e.g., depth, wellbore geometry, temperature, pressure, and formation composition.
A casing profile includes multiple casing strings, such as conductor casing, surface casing, intermediate casing and production casing. Furthermore, each casing string undergoes a cement job. The large-diameter conductor casing protects shallow formations from contamination by drilling fluid and helps prevent washouts involving unconsolidated topsoils and sediments. Surface casing, the second string, has a smaller diameter, maintains borehole integrity and prevents contamination of shallow groundwater by hydrocarbons, subterranean brines and drilling fluids. The intermediate casing isolates hydrocarbon-bearing, abnormally pressured, fractured and lost circulation zones, providing well control as engineers drill deeper. Multiple strings of intermediate casing may be required to reach the target producing zone. The production casing, or liner, is the last and smallest tubular element in the well. It isolates the zones above and within the production zone and withstands all of the anticipated loads throughout the well's life.
Once the wellbore is lined with a casing, a cement head is installed above the rig floor to introduce cement into the well. The cement is pumped thru various cement lines traveling thru the cement head and into the well. During the cementing operation, the casing is typically supported by the rig hook and travelling block or the rotary table of the drilling rig. Furthermore, performing the cement job in oil and gas wells typically requires the drill-rig to be standing above the well during the operation for a fair amount of time (which can be up to a few days of non-productive time).
When multiple wells are drilled on a pad (land application), the drilling sequence may be based on batch drilling. With such process, the corresponding sections of all wells are drilled in series, requiring the rig to be moved from well to well after each section. Typically, a well section is drilled; then the casing is lowered into the well-bore and then cemented. The rig is not moved until after the cement has set, causing significant non-productive time.